Apparatus and method for well logging and data processing device

ABSTRACT

The disclosure relates to an apparatus and a method for well logging as well as a data processing device thereof. Said apparatus for well logging comprises a drill collar body and an array of antennas, wherein said array of antennas comprises at least a pair of transmitting antenna and receiving antenna, said transmitting antenna and receiving antenna are configured for generating a curve of axial forward depth of investigation. By employing the method for well logging according to the present invention, not only the variation of resistivity in the axial forward formation may be measured in real time during the drilling, but also the interfacial characteristics of the axial forward formations having different resistivity may be discriminated during drilling.

TECHNICAL FIELD

The invention relates to the technical field of well logging, morespecifically, the invention relates to the technical field ofmeasurement while drilling (i.e. MWD) in the drilling industry. Inparticular, the invention relates to an apparatus and method for welllogging as well as a data processing device thereof.

BACKGROUND OF THE INVENTION

At present, in the art of MWD in the drilling industry such as theexplorating of oil and gas, coalbed methane, shale gas trapped withinshale formations, drilling, mining and so on, the formation resistivityis generally used to form stratigraphic profiles and to determine oilsaturation, gas content from coal structure and mineral fractures ofreservoirs, thus the formation resistivity is a primary basis forexplaining and evaluating oil and gas, coal, mineral reserves in welllogging. The currently known logging-while-drilling (LWD) resistivitylogging technique includes LWD lateral resistivity logging, LWDelectromagnetic wave propagation resistivity logging and LWD inductionresistivity logging.

The principle of the LWD lateral resistivity logging primarily involvesproviding current by power supply electrodes, forming an electricalfield in formations around a borehole, measuring the distribution of theelectrical field in the formations and obtaining the formationresistivity. An apparatus for LWD lateral resistivity logging makes thedrill bit as an electrode, and also may employ a loop electrode andthree button electrodes approximate the drill bit to take resistivitymeasurement. In the case where a drill bit serves as an electrode,before mud invasion or possible damage to the borehole, the apparatusfor LWD lateral resistivity logging may measure the resistivity of athin layer of 5-10 cm. While if a three-button electrode array isemployed, a high-resolution lateral resistivity measurement may beachieved, which may reduce influence of surrounding rocks, and mayprovide a true formation resistivity response even in brine mud orformations having a high resistivity. Besides, if a loop electrode isemployed, the resistivity information within a 360° range around theborehole may be obtained.

However, the above-mentioned apparatus for LWD lateral resistivitylogging has deficiencies as follows: since the technique of lateralresistivity logging belongs to the method of DC electrical logging, thatis to say, it needs to have a power supply electrode to conduct DCcurrent into a formation, and then use a measuring electrode to measurethe electrical potential at certain point in the well, thus such lateralresistivity logging method may be used only when there is conductive mudin the well that provides current channels. However, during thepractical drilling operation such as the oil drilling operation,sometimes, in order to obtain the information of original oil saturationin a formation, it needs to employ oil-based mud drilling, or evenemploy air drilling. In such cases, the DC electrical logging methodcannot be used, that is, the method of LWD lateral resistivity loggingis no longer applicable in said cases.

An apparatus for LWD electromagnetic wave propagation resistivitylogging employs multi-coil system, the propagation frequency is 1-8 MHz,the coil system is based on the body structure of the drill collar, andthe coil system is wound around the drill collar. A phase shift shallowresistivity and an attenuation deep resistivity are calculated throughmeasuring the amplitude ratio or phase difference between differenttransmitting coil and receiving coil and thereafter converting theamplitude ratio or phase difference to apparent resistivity of theformation. In ideal cases, the axial resolution of the apparatus for LWDelectromagnetic wave propagation resistivity logging is dependent on theinterval between two receiving coils, and the measurement data atmultiple depths of investigation may be used to explain the status ofmud invasion. Normally, those skilled in the art will appreciate thatthe depth of investigation of the phase resistivity is less, while thedepth of investigation of the attenuation resistivity is deeper.

A Chinese patent application publication No. CN101609169A titled “Methodfor improving the precision of electromagnetic wave resistivitymeasurement and expanding measurement range thereof” discloses that themutual induction electromotive force which are not related to theresistivity of formation, zero signals of a circuit and base signals ofan antenna system in a plot of amplitude attenuation-resistivityconversion and a plot of phase difference-resistivity conversion of themutual induction electromotive force is eliminated by calculating themutual induction electromotive force between a transmitting antenna anda receiving antenna, and the conversion of phase difference andamplitude attenuation to the resistivity of formation may be obtained.

Besides, the reference document titled “Basic theory of an apparatus ofelectromagnetic wave resistivity LWD with tilted antennas and theapplication for geo-steering thereof” published in Journal of ChinaUniversity of Petroleum calculates the response of the apparatus ofelectromagnetic wave resistivity LWD with tilted antennas by using arecursive matrix method for computation of the Green's function ofmagnetic dipole source in anisotropic horizontally stratified medium,analyzes the influence of the relative inclination of the borehole andthe dip angle of the receiving coil upon the amplitude ratio and phasedifference of the receiving signal, as well as the characteristics ofthe conventional tools and novel tools of making response to the curvehorn in the direction perpendicular to the axis of the tool, wherebypredicting the existence of a formation boundary earlier.

However, although the various apparatuses for LWD electromagnetic wavepropagation resistivity logging at present may measure resistivity atdifferent depths of investigation, they have deficiencies as follows.

Firstly, the signal frequency used by the apparatus for LWDelectromagnetic wave propagation resistivity logging is too high, thedepth of investigation is limited due to the propagation effect of theelectromagnetic wave.

Secondly, the measurement result of the apparatus for LWDelectromagnetic wave propagation resistivity logging will be influencedby geological factors, especially influenced by surrounding rocks,because the measurement result of the apparatus is not only limited tothe formation area between receiving coils, but also related toparameters of the whole formation between the transmitting coils andreceiving coils, and even the formation within a relatively small areaaround the transmitting coil will influence the measurement result.Therefore, the axial resolution of the apparatus for well loggingdepends largely on the resistivity of the formation in which the wholeapparatus is located.

Thirdly, since the coil system of the apparatus for LWD electromagneticwave propagation resistivity logging is wound on the surface of thedrill collar, the manufacture process thereof is rather complicated.Moreover, the coil system may be easily abraded and thus be damagedduring operating. Then, when the size of the borehole varies, it needsto rewind the coils, thus the maintenance and overhaul is rathercomplicated and the maintenance cost is high. Besides, similar to theapparatus for LWD lateral resistivity logging, the apparatus for LWDelectromagnetic wave propagation resistivity logging is unable to workin oil-based mud.

An apparatus for LWD induction resistivity logging applies the principleof electromagnetic induction. When alternating current at constantamplitude and frequency is applied in a transmitting coil, eddy currentis induced in the formation surrounding said coil, and the eddy currentper se will form a secondary alternating electromagnetic field. Underthe effect of the secondary alternating electromagnetic field, inducedelectromotive force will be generated in receiving coils. The amount ofsaid electromotive force is associated with the conductivity offormation, and the resistivity of formation may be obtained throughmeasuring the induced electromotive force.

The coil system of the apparatus for LWD induction resistivity loggingat present employs one transmitting coil and two receiving coils, andone of said two receiving coils is the primary receiving coil while theother one is the compensatory coil. The coil system is positioned in aV-shaped groove with a reflection layer on a lateral face of the drillcollar. The response of well logging is sensitive to the resistivityvariation of the formation in the front area of the V-shaped groove,thus it has the characteristic of directional measurement. The apparatusfor LWD induction resistivity logging is supplied with power by abattery. On the top of the battery, there is provided with a male bucklejoint which may be joined to a female buckle joint on the bottom of theapparatus for LWD induction resistivity logging for transferringreal-time data from the apparatus for LWD induction resistivity loggingto the surface. The same survey sub may be adapted to the requirementsof boreholes in different sizes.

The advantages of such an apparatus for LWD induction resistivitylogging are as follows. Since the signal frequency thereof is 20 kHz,which is greatly lower than the frequency of a high-frequency apparatus,it is not easily absorbed by formations. Furthermore, the depth ofinvestigation is deep and the range of measurement is relatively large,which may reach 0.1-1000 Ωm. Moreover, the structure of such anapparatus is simple, and one survey sub may be adapted to therequirements of boreholes in different sizes. Also, the maintenance andoverhaul is relatively easy, and it is adapted to different drillingfluids.

However, such an apparatus for LWD induction resistivity logging furtherhas deficiencies as follows. Since the apparatus employs a coil systemcomposed of one transmitting coil and two receiving coils and having asingle fixed depth of investigation, said apparatus may only provide theresistivity of formation in one radial depth of investigation, whilecannot be used to explain complicated invasive profile and to separatethe corrosive formations. Besides, as for a corrosive formation, mudinvasion causes the resistivity thereof to vary in radial directions,since only a resistivity value in one radial depth of investigation canbe obtained at a measurement point in the same depth, the apparatus forLWD induction resistivity logging cannot be used to explain the invasioncondition of the formation, and the condition where the formation isinvaded by mud and the reservoir permeability cannot be determined. Thisis disadvantageous for explanation of oil and gas reservoirs, thus itcannot be used to calculate the true formation resistivity accurately.Furthermore, as for different types of mud invasion and the resistivityin different radial depths of investigation, the characteristics of theoil-gas-water layers are different. The oil and gas may be identifiedaccording to different degrees of mud invasion influences upon multipleresistivity curves at different depths of investigation, as well asdifferential characteristics manifested by the oil-gas-water layers.Therefore, multi-depth resistivity measurement is significant to a LWDapparatus. However, the apparatus for LWD induction resistivity loggingat present is unable to meet the requirement. Furthermore, since thedesigned structure for the coil system of said apparatus is fixed, eachcoil system may only measure the resistivity at one depth, and differentcoil systems have to be used to take multiple measurements in order toobtain resistivity at different depths of investigation. As a result, itis hard to carry out such a LWD induction resistivity logging mannerduring the practical application.

In summary, no matter which one of said LWD resistivity apparatuses isconcerned, it has many deficiencies. Moreover, each of said LWDresistivity apparatuses is only dedicated to measure and calculate theradial depth of investigation, while does not mention or refer to themeasurement of axial-forward depth of investigation. However, as thenumber of the transmitting antennas and receiving antennas of variousapparatuses for LWD resistivity logging increases continuously, thetransmitting frequency decreases. Therefore, the axial depthinvestigation becomes growingly important to the drill engineering.Consequently, the need for the method of LWD axial-forward investigationis growingly increased in the art of well drilling and logging.

SUMMARY OF THE INVENTION

In order to overcome the aforesaid one or more deficiencies existing inthe prior techniques of LWD resistivity logging, the invention providesa new method for logging while drilling, which may not only measure inreal time the variation of resistivity in the axial forward formationduring drilling, but also discriminate the interfacial characteristicsof the axial forward formations having different resistivity duringdrilling.

According to one respect of the invention, a method for well logging isprovided, comprising:

(a) a step of selecting homogeneous measurement point, wherein anapparatus for well logging is configured to select two sequentialmeasurement points to take at least two sequential measurements;(b) determining whether said selected two sequential measurement pointsmay serve as selectable points of homogeneous formation according to themeasurement results at said two sequential measurement points, if yes,then proceeds to the following step (c);(c) deriving the amplitude ratio essential value and phase differenceessential value of the signal response generated by the apparatus forwell logging, which corresponds to the formation resistivity of themeasured high-resistivity target formation, from said two selectablepoints of homogeneous formation;(d) deriving the amplitude ratio standard value and phase differencestandard value corresponding to the formation resistivity of themeasured target formation from said amplitude ratio essential value andphase difference essential value;(e) setting a layer-out threshold of the formation for said measuredhigh-resistivity target formation according to said amplitude ratiostandard value and phase difference standard value.(f) selecting a next measurement point to take at least two measurementsat said next measurement point;(g) deciding whether the amount of variation in amplitude ratio and/orthe amount of variation in phase difference of the induced electromotiveforce between a pair of receiving antennas of the apparatus for welllogging at the current measurement point are greater than said layer-outthreshold; if yes, then proceed to the following step (h);(h) it is determined that a formation with low-resistivity appears infront of the apparatus for well logging.

According to another respect of the invention, a data processing deviceis provided, wherein said data processing device comprises:

means for determining selectable points of homogeneous formation, whichis configured to determine whether both of the two sequentialmeasurement points currently selected by an apparatus for well loggingmay serve as selectable points of homogeneous formation or not;means for deriving essential values, which is configured to derive theamplitude ratio essential value and phase difference essential value ofthe signal response generated by the apparatus for well loggingaccording to said two selectable points of homogeneous formation when itis determined by said means for determining selectable point ofhomogeneous formation that the two sequential measurement points mayserve as selectable points of homogeneous formation; wherein theamplitude ratio essential value and phase difference essential valuecorrespond to the formation resistivity of the measured high-resistivitytarget formation;means for deriving standard values, which is configured to derive theamplitude ratio standard value and phase difference standard valuecorresponding to the formation resistivity of the measuredhigh-resistivity target formation from said amplitude ratio essentialvalue and phase difference essential value,means for setting layer-out threshold, which is configured to set alayer-out threshold of the formation for said measured high-resistivitytarget formation according to said amplitude ratio standard value andphase difference standard value;means for selecting the third through the n^(th) measurement points andcalculating the amount of variation in amplitude ratio and phasedifference, which is configured to select a next measurement point totake at least two measurements at said next measurement point, andcalculate the amount of variation in amplitude ratio and the amount ofvariation in phase difference of the induced electromotive force betweena pair of receiving antennas of the apparatus for well logging at thecurrent selected measurement point; andmeans for determining the presence of the formation withlow-resistivity, which comprises a unit for determining the occurrenceof layer-out, said unit is configured to decide whether the amount ofvariation in amplitude ratio and/or the amount of variation in phasedifference at the current selected measurement point are greater thansaid layer-out threshold; if yes, it is determined that a formation withlow-resistivity appears in front of the apparatus for well logging.

According to a yet another aspect of the invention, an apparatus forwell logging is provided, wherein said apparatus for well loggingcomprises a drill collar body and an array of antennas, wherein saidarray of antennas comprises at least a pair of transmitting antenna andreceiving antenna, said transmitting antenna and receiving antenna areconfigured for generating a curve of axial forward depth ofinvestigation.

Compared to the radial-depth investigation, the axial-depthinvestigation according to the invention possesses the followingadvantages.

First of all, the axial-depth investigation according to the inventionmay effectively control the trajectory of the inclination-made sectionin drill engineering. The well known measurement of horizontal segmentof formation usually assumes a horizontal layered distribution, whendeflection begins, the apparatus for resistivity logging is almostperpendicular to the horizontal layered formations. Therefore, theresponse of radial investigation can only reflect the variation inresistivity of the measured formation at certain layer, while theresponse of axial investigation has a plurality of axial layers ofinvestigation, which may reflect the variation in resistivity of themeasured formation at multiple different drilling depths, effectivelyidentify formation boundary and oil-water contact, and adjust thedeflection radian to make it accurate and smooth, thus improve thedrilling quality in the inclination section.

Further, when the drilling bit enters a complicated highly-deviated wellor horizontal section, the axial-depth investigation according to theinvention may perform axial investigation at different depths on theformations along the direction of drilling movement. Therefore, it ismore direct and accurate as compared to the method of radialinvestigation, and it can predetermine a thin reservoir, a complicatedfolded and interlaid bed, whereby effectively keeping away from faults,drilling to a long distance along high-dip reservoirs and obtaining ahighest effective drilling catching rate for oil and gas.

The method for well logging according to the invention and thecorresponding data processing apparatus may measure in real time thevariation characteristics of the variation ratio of the formationresistivity in the drilling process, discriminate in real time theformation boundary and oil-water interface, and capture the bestopportunity of entering the oil and gas reservoir. Moreover, the methodmay predict the geological information in front of the drill bit muchearlier, adjust the trajectory of the borehole timely in high-dip andanisotropic-formation horizontal wells, and control the drill tool topass through the best location of the petroleum reservoir so as toobtain the largest oil contact face. Therefore, it is extremely adaptiveto geo-steering in petroleum engineering.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an apparatus for well logging according to a preferredembodiment of the invention.

FIG. 2 shows a model diagram of a two-layered formation employed by amethod for well logging according to the invention.

FIG. 3 illustrates a diagram showing a relationship where the amplituderatio response of the formation with a resistivity contrast of 10/1varies with the position of the formation boundary.

FIG. 4 illustrates a diagram showing a relationship where the phasedifference response of a formation with a resistivity contrast of 10/1varies with the position of the formation boundary.

FIG. 5 illustrates a diagram showing a relationship where the amplituderatio response of a formation with a resistivity contrast of 50/1 varieswith the position of the formation boundary.

FIG. 6 illustrates a diagram showing a relationship where the phasedifference response of a formation with a resistivity contrast of 50/1varies with the position of the formation boundary.

FIG. 7 illustrates a diagram showing a relationship where the amplituderatio response of a formation with a resistivity contrast of 200/1varies with the position of the formation boundary.

FIG. 8 illustrates a diagram showing a relationship where the phasedifference response of a formation with a resistivity contrast of 200/1varies with the position of the formation boundary.

FIG. 9 shows a reference table for eigenvalues of the resistivity, theamplitude ratio and the phase difference of various types of formationsgenerated by the antenna system T2-R1-R2 of an apparatus for welllogging according to a preferred embodiment of the invention at thefrequency of 2 MHz.

FIG. 10 shows a reference table for eigenvalues of the resistivity, theamplitude ratio and the phase difference of various types of formationsgenerated by the antenna system T2-R1-R2 of an apparatus for welllogging according to a preferred embodiment of the invention at thefrequency of 400 kHz.

FIG. 11 shows a reference table for eigenvalues of the resistivity, theamplitude ratio and the phase difference of various types of formationsgenerated by the antenna system T1-R1-R2 of an apparatus for welllogging according to a preferred embodiment of the invention at thefrequency of 2 MHz.

FIG. 12 shows a reference table for eigenvalues of the resistivity, theamplitude ratio and the phase difference of various types of formationsgenerated by the antenna system T1-R1-R2 of an apparatus for welllogging according to a preferred embodiment of the invention at thefrequency of 400 kHz.

FIG. 13 shows a flowchart of a method for logging while drillingaccording to a preferred embodiment of the invention.

FIG. 14 shows a block diagram of a device for processing well-loggingdata according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Some terms are used for denoting specific system components throughoutthe application document. As would be appreciated by those skilled inthe art, different designations may usually be used for denoting thesame component, thus the application document does not intend todistinguish those components that are only different in name rather thanin function. In the application document, terms “comprise”, “include”and “have” are used in the opening way, and thus they shall be construedas meaning “comprise but not limited to . . . ”. Besides, Terms“substantially”, “essentially”, or “approximately”, that may be usedherein, relate to an industry-accepted tolerance to the correspondingterm. The term “coupled”, as may be used herein, includes directcoupling and indirect coupling via another component, element, circuit,or module where, for indirect coupling, the intervening component,element, circuit, or module does not modify the information of a signalbut may adjust its current level, voltage level, and/or power level.Inferred coupling, for example where one element is coupled to anotherelement by inference, includes direct and indirect coupling between twoelements in the same manner as “coupled”.

In the following description, for the purpose of explanation, manyspecific details are set forth so as to provide a thorough understandingof the invention. However, it is apparent for those skilled in the artthat the apparatus, method and device of the present invention may beimplemented without those specific details. The reference to the“embodiment”, “example” or similar language in the Description meansthat the specific features, structures or characteristics described inconnection with the embodiment or example are comprised in at least saidembodiment or example, but are not necessarily comprised in otherembodiments or examples. Various instances of the phrases of “in anembodiment”, “in a preferred embodiment” or similar phrase in differentportions of the Description do not necessarily all refer to the sameembodiment.

The present invention is further illustrated in connection withpreferred embodiments and corresponding figures below.

FIG. 1 shows an apparatus for well logging according to a preferredembodiment of the invention. In such a preferred embodiment, theapparatus for well logging is an apparatus for electromagnetic wavepropagation resistivity logging which comprises a drill collar body 12,an array of antennas 7-11, 13-15, an internal electronic circuit (notshown in the figure) and a solidifying sealing member for coupling eachcomponent.

As shown in FIG. 1, the drill collar body 12 is preferably made of acylindrical stainless steel material with an axial via hole therein inthe embodiment. A plurality of grooves that are preferably circle-shapedor ellipse-shaped are carved on the exterior surface of the drill collarbody 12, and said grooves are used for installation of the transmittingantenna(s) or receiving antenna(s).

In the preferred embodiment shown in FIG. 1, the array of antennacomprises four transmitting antennas T1 (as shown by reference number11), T2 (as shown by reference number 14), T3 (as shown by referencenumber 13) and T4 (as shown by reference number 15), and four receivingantennas R1 (as shown by reference number 7), R2 (as shown by referencenumber 8), R3 (as shown by reference number 9) and R4 (as shown byreference number 10).

As shown in FIG. 1, the transmitting antennas and the receiving antennasare installed from the left side of FIG. 1 to the right side (i.e. fromthe drill-collar-tail to the drill-head of the drill collar body 12)preferably according to the sequence of the receiving antenna R3, thetransmitting antenna T3, the transmitting antenna T1, the receivingantenna R1, the receiving antenna R2, the transmitting antenna T2, thetransmitting antenna T4, and the receiving antenna R4.

In the preferred embodiment, the middle point between the receivingantennas R1 and R2 is the measurement point, and the transmittingantennas T1, T2, T3 and T4 are preferably installed symmetrically aboutsaid measurement point. The receiving antennas R1 and R2 preferably area pair of receiving antennas having a installation angle of 0°, and thereceiving antennas R3 and R4 are another pair of receiving antennassymmetrical about said measurement point, as shown in FIG. 1. Thereceiving antennas R3 and R4 are preferably positioned on the two endsof the drill collar. The installation angles of the receiving antennasR3 and R4 may be set in any appropriate manner, and they are preferably(but not limited to be) set to 45° and −45° in the preferred embodiment.

With respect to any one of the transmitting antennas and any pair ofreceiving antennas (e.g. transmitting antenna T1, receiving antennas R1and R2), electromagnetic signals propagate via the surrounding formationand the drill collar body when the transmitting antenna is excited. Theelectromagnetic signals are reflected and transmitted by the formation,and produce electromagnetic induction signals on the receiving antennas.The electromagnetic induction signals are collected by the receivingantennas, amplified and filtered by the internal electronic circuit, andfinally transformed to the function of the resistivity of the formationthrough which they propagates.

In the case of the apparatus for well logging (e.g. the apparatus forelectromagnetic wave propagation resistivity logging) being operateddown hole, if the electrical parameter of the formation (e.g. theresistivity contrast of the formations) in front of said apparatus issubstantially constant, it indicates that no formation boundary appears.At this time, the electromagnetic signal reflected onto the receivingantennas is substantially constant. On the contrary, if the electricalparameter of the formation in front of said apparatus varies, itindicates that a formation boundary appears. At this time, theelectromagnetic signal reflecting onto the receiving antennas varies,thus a signal difference is generated. The distance of the axial forwardinvestigation may be obtained by continuously collecting and calculatingthe signal difference.

Any combination of any one of the transmitting antennas with any pair ofreceiving antennas in the apparatus for well logging according to theinvention may generate a curve of axial forward investigation. Bycomparing and processing all of the curves of axial forwardinvestigation, environmental influence (e.g. influence of the borehole)and measurement error may be eliminated, whereby the axial forwardinvestigation accuracy of the apparatus for well logging may beimproved.

Next, a preferred method for logging while drilling according to anotherembodiment of the invention will be described in detail in connectionwith the figures.

As shown in FIG. 13, a method for logging while drilling according to apreferred embodiment of the invention, e.g. a method for electromagneticwave propagation resistivity axial forward logging, comprises steps asfollows.

In step 1301, an apparatus for logging while drilling (e.g. theapparatus for electromagnetic wave propagation resistivity axial forwardlogging as shown in FIG. 1) are placed into a target formation withhigh-resistivity at a certain depth. The apparatus for well logging takemeasurements continually during drilling, and the direction of theinvestigation is consistent with the direction of the axial movement ofsaid apparatus for well logging.

In step 1302, two sequential measurement points (e.g. the firstmeasurement point and the second measurement point) are selected, and atleast two sequential measurements are took at each measurement point.

In step 1303, if it can be determined from the at least two sequentialmeasurements at the first measurement point that, the amount ofvariation in amplitude ratio ΔAtt and the amount of variation in phasedifference ΔPSD of the induced electromotive force between the firstreceiving antenna and the second receiving antenna along the axialdirection of the apparatus for well logging are within their respectivepreset threshold range, the first measurement point is stored as thefirst selectable point of homogeneous formation. For example, the presetthreshold range for the amount of variation in amplitude ratio may be0-0.03 dB or other appropriate preset range, and the preset thresholdrange for the amount of variation in phase difference may be 0°-0.1° orother appropriate preset range,

In step 1304, if it can be determined from the at least two sequentialmeasurements at the second measurement point that, the amount ofvariation in amplitude ratio ΔAtt and the amount of variation in phasedifference ΔPSD of the induced electromotive force between the firstreceiving antenna and the second receiving antenna along the axialdirection of the apparatus for well logging are within their respectivepreset threshold range, the second measurement point is stored as thesecond selectable point of homogeneous formation.

If it is determined in steps 1303 and 1304 that any one or both of thefirst and second measurement points do not meet the aforesaidrequirements, then returning to step 1302 to continue to take furthermeasurements while drilling and select another two sequentialmeasurement points, until both of the two currently-selected measurementpoints meet the aforesaid requirements.

After the first and second selectable points of homogenous formation aredetermined via steps 1303 and 1304, in step 1305, the average value ormean square root of the multiple measurements of amplitude ratio of theinduced electromotive force between the first receiving antenna and thesecond receiving antenna measured at both of the first and secondselectable points of homogeneous formation is considered as theamplitude ratio essential value Δtt0 of the signal response generated bythe apparatus for well logging, which corresponds to the formationresistivity of the measured target formation. In a similar way, theaverage value or mean square root of the multiple measurements of thephase difference measured at both of the first and second selectablepoints of homogeneous formation is considered as the phase differenceessential value PSD0 corresponding to the formation resistivity of themeasured target formation.

Next, in step 1306, deriving and storing the standard valuecorresponding to the formation resistivity of the measured targetformation. In particular, said amplitude ratio essential value Att0 andphase difference essential value PSD0 of the measured high-resistivitytarget formation are compared with the corresponding predeterminedeigenvalues of various types of formations, then the eigenvalues of thetype of formation closest to said amplitude ratio essential value Att0and phase difference essential value PSD0 can be selected as theamplitude ratio standard value and phase difference standard valuecorresponding to the formation resistivity of the measuredhigh-resistivity target formation. The amplitude ratio standard valueand phase difference standard value are stored in a memory.

Optionally, in step 1307, a layer-out threshold of said measuredhigh-resistivity target formation is set according to the amplituderatio standard value and phase difference standard value correspondingto the formation resistivity of the measured high-resistivity targetformation. Specifically, when the apparatus for well logging approachesa formation boundary with low-resistivity, the amplitude ratio and phasedifference of the induced electromotive force between the firstreceiving antenna and the second receiving antenna in the axialdirection of said apparatus for well logging will vary. The closer theapparatus for well logging approaches the boundary with low-resistivity,the greater the amount of variation in said actually measured amplituderatio and phase difference with respect to the amplitude ratio standardvalue and the phase difference standard value are. When the amount ofvariation in the amplitude ratio and phase difference reaches or exceedsa preset value, it is normally deemed that a formation withlow-resistivity appears in front of the apparatus for well logging. Saidpreset value is named as the layer-out threshold herein.

It is noted that the layer-out threshold for different measuredformations may be set to different preset values by those skilled in theart according to the characteristics of the actually measured formationsand measurement conditions. Generally, the layer-out threshold may bederived from the resistivity contrast between the two formations of thecurrently measured formation and the axial forward formation.Preferably, no matter how the resistivity contrast between the twoformations of the currently measured formation and the axial forwardformation is like, the layer-out threshold may be set to be 1%-30% ofthe amplitude ratio standard value or phase difference standard value.Further preferably, when said resistivity contrast is 1/10, thelayer-out threshold may be preferably set to be 10% of said amplituderatio standard value or phase difference standard value. Theaforementioned manner of determining the layer-out threshold and thespecific value of the layer-out threshold are for illustrative purposeonly rather than for restrictive purpose, and those skilled in the artmay select appropriate values in other ways according to practice.

As shown in FIG. 13, in step 1308, proceeding to select a nextmeasurement point, take at least two measurements at the nextmeasurement point, and calculate the amount of variation in amplituderatio ΔAtt and the amount of variation in phase difference ΔPSD of theinduced electromotive force between the first receiving antenna and thesecond receiving antenna along the axial direction of the apparatus forwell logging at such a measurement point.

In step 1309, determining whether the amount of variation in amplituderatio ΔAtt and the amount of variation in phase difference ΔPSDcalculated in step 1308 are greater than the layer-out threshold. Ifyes, it is determined that a formation with low-resistivity appears infront of said apparatus for well logging. If not, storing the currentamount of variation in amplitude ratio ΔAtt and the amount of variationin phase difference ΔPSD, and then determining whether the currentlyselected measurement point is the preset nth measurement point, if not,returning to step 1308 and proceeding to select a next measurement pointas well as performing the calculation of the amount of variation inamplitude ratio ΔAtt and the amount of variation in phase differenceΔPSD; or else if the currently selected measurement point is the presetnth measurement point, then moving to step 1310.

Please note that said number ‘n’ is preset by those skilled in the artbased on the characteristics of the measured formation and the measuringspeed. For example, if the measured formation is a softer formation(e.g. sand rocks in a coastal area), n can be smaller, while if themeasured formation is a harder formation (e.g. shale rocks), n can belarger. Typically, for a general formation, n may be preferably presetto be 20-30, but the invention never to be limited to such a range ofvalue, and other appropriate values may be preset for n.

In step 1310, the tendency of variation in the amplitude ratio and thetendency of variation in the phase difference are determined accordingto the previously stored amount of variation in amplitude ratio ΔAtt andamount of variation in phase difference ΔPSD at each measurement point.

If the tendency of variation is that the amount of variation inamplitude ratio and the amount of variation in phase difference maintaina progressive increase from the third measurement point to the n^(th)measurement point (i.e. the amount of variation in amplitude ratio andthe amount of variation in phase difference at the (m+1)^(th)measurement point are greater than the amount of variation in amplituderatio and the amount of variation in phase difference at the m^(th)measurement point, wherein m=1, 2, . . . , n−1), it is determined that aformation with low-resistivity appears in front of said apparatus forwell logging.

Otherwise, if the tendency of variation is that the amount of variationin amplitude ratio and the amount of variation in phase differencemaintain an approximatively progressive increase from the thirdmeasurement point to the n^(th) measurement point, it is also determinedthat a formation with low-resistivity appears in front of said apparatusfor well logging. As appreciated by those skilled in the art, the‘approximatively progressive increase’ herein means that, although thereare some ripples in the tendency of variation (in other words, theamount of variation in amplitude ratio and the amount of variation inphase difference at certain measurement point are smaller than theamount of variation in amplitude ratio and phase difference at theimmediately previous measurement point), there are for example at least70% of the measurement points to maintain the tendency of progressiveincrease. Said percentage also can be preset by those skilled in the artaccording to practice, and the percentage of 70% is only forillustrative rather than for restrictive.

Or else, if the tendency of variation does neither maintain aprogressive increase, nor maintain an approximatively progressiveincrease, it is determined that no formation with low-resistivityappears in front of said apparatus for well logging.

According to another preferred embodiment of the present invention,during said process of deriving essential value in the step 1305, theformation resistivity, amplitude ratio and phase difference of the firstand second selectable points of homogenous formation can be calculatedby dyadic Green's function of magnetic dipole source with method ofrecursive matrix. For example, FIGS. 9-12 show several exemplaryreference tables for eigenvalues of the resistivity, amplitude ratio andphase difference of various types of formations, and the correspondingphysical quantities in said reference tables are calculated by dyadicGreen's function of magnetic dipole source with method of recursivematrix.

It can be seen that FIG. 9 shows a reference table for eigenvalues ofthe resistivity, the amplitude ratio and the phase difference of varioustypes offormations generated by the antenna system T2-R1-R2 of anapparatus for well logging according to a preferred embodiment of theinvention at the frequency of 2 MHz. FIG. 10 shows a reference table foreigenvalues of the resistivity, the amplitude ratio and the phasedifference of various types of formations generated by the antennasystem T2-R1-R2 of an apparatus for well logging according to apreferred embodiment of the invention at the frequency of 400 kHz. FIG.11 shows a reference table for eigenvalues of the resistivity, theamplitude ratio and the phase difference of various types of formationsgenerated by the antenna system T1-R1-R2 of an apparatus for welllogging according to a preferred embodiment of the invention at thefrequency of 2 MHz. FIG. 12 shows a reference table for eigenvalues ofthe resistivity, the amplitude ratio and the phase difference of varioustypes of formations generated by the antenna system T1-R1-R2 of anapparatus for well logging according to a preferred embodiment of theinvention at the frequency of 400 kHz.

Furthermore, according to a further preferred embodiment of the presentinvention, said method for well logging further preferably comprises astep of calculating the distance from an axial forward formation withlow-resistivity to the current measurement point of the apparatus forwell logging additionally by using Sommerfeld integrals.

FIG. 2 illustrates a diagram showing a two-layered formation model usedby the method and apparatus for well logging according to the presentinvention.

As shown in FIG. 2, the reference number of 1 represents a formation 1;the reference number of 2 represents another formation 2; the referencenumber of 3 represents a formation boundary between formation 1 andformation 2; the reference number of 4 represents a mandrel axis of theapparatus for electromagnetic wave propagation resistivity axial forwardlogging; the reference number of 5 represents a measurement point of theapparatus for electromagnetic wave propagation resistivity axial forwardlogging; the reference number of 6 represents the distance from themeasurement point 5 to the formation boundary 3; the reference number of7 represents a receiving antenna R1 at an installation angle of zerodegree; the reference number of 8 represents a receiving antenna R2preferably at an installation angle of zero degree; the reference numberof 9 represents a receiving antenna R3 preferably at an installationangle of 45°; the reference number of 10 represents a receiving antennaR4 preferably at an installation angle of −45°; the reference number of11 represents a transmitting antenna T1 preferably at an installationangle of zero degree.

According to said two-layered formation model, said apparatus forelectromagnetic wave propagation resistivity axial forward logging isdisposed in the formation 1 and is perpendicular to the formationboundary 3 between the formation 1 and the formation 2. The variationsin the amplitude ratio and phase difference in formations havingdifferent resistivity contrasts may be obtained through varying thedistance from the formation boundary 3 to the center point of theapparatus.

FIG. 3 illustrates a diagram showing a relationship where the amplituderatio response of the formation with a resistivity contrast of 10/1varies with the position of the formation boundary. FIG. 4 illustrates adiagram showing a relationship where the phase difference response of aformation with a resistivity contrast of 10/1 varies with the positionof the formation boundary. FIG. 5 illustrates a diagram showing arelationship where the amplitude ratio response of a formation with aresistivity contrast of 50/1 varies with the position of the formationboundary. FIG. 6 illustrates a diagram showing a relationship where thephase difference response of a formation with a resistivity contrast of50/1 varies with the position of the formation boundary. FIG. 7illustrates a diagram showing a relationship where the amplitude ratioresponse of a formation with a resistivity contrast of 200/1 varies withthe position of the formation boundary. FIG. 8 illustrates a diagramshowing a relationship where the phase difference response of aformation with a resistivity contrast of 200/1 varies with the positionof the formation boundary.

In FIG. 3-FIG. 8, the x-axis represents the distance from the formationboundary 3 to the center point of the apparatus, and the y-axisrepresents the difference of the signal response generated by theantenna array disposed in the two-layered formation and in thehomogenous formation with the resistivity of the formation 1.

Assuming that the threshold for amplitude ratio of the apparatus forelectromagnetic wave propagation resistivity axial forward loggingaccording to a preferred embodiment of the invention is 0.02 dB and thethreshold for phase difference is 0.1° (as shown by the transverse linein FIG. 3-FIG. 8), it can be seen from FIG. 3-FIG. 8 the axial depth ofinvestigation of respective pair of antennas of the apparatus for welllogging disposed in various formations having different resistivitycontrasts.

For example, in the formation having a resistivity contrast of 10/1, ifthe frequency of the pair of transmitting and receiving antenna is 2MHz, the axial depth of investigation for the amplitude ratio and phasedifference of the pair of antennas with 16/22 inch are 41 inch and 26inch respectively, and the axial depth of investigation for theamplitude ratio and phase difference of the pair of antennas with 32/38inch are 56 inch and 37 inch, respectively. If the frequency of the pairof transmitting and receiving antenna is 400 kHz, the axial depth ofinvestigation for the amplitude ratio and phase difference of the pairof antennas with 16/22 inch are 43 inch and 35 inch respectively, andthe axial depth of investigation for the amplitude ratio and phasedifference of the pair of antenna with 32/38 inch are 67 inch and 48inch, respectively.

In the formation having a resistivity contrast of 50/1, if saidfrequency is 2 MHz, the axial depth of investigation for the amplituderatio and phase difference of the pair of antennas with 16/22 inch are55 inch and 35 inch respectively, and the axial depth of investigationfor the amplitude ratio and phase difference of the pair of antennaswith 32/38 inch are 77 inch and 46 inch respectively. If said frequencyis 400 kHz, the axial depth of investigation for the amplitude ratio andphase difference of the pair of antennas with 16/22 inch are 49 inch and44 inch respectively, and the axial depth of investigation for theamplitude ratio and phase difference of the pair of antennas with 32/38inch are 82 inch and 62 inch, respectively.

In the formation having a resistivity contrast of 200/1, if saidfrequency is 2 MHz, the axial depth of investigation for the amplituderatio and phase difference of the pair of antennas with 16/22 inch are61 inch and 43 inch respectively, and the axial depth of investigationfor the amplitude ratio and phase difference of the pair of antennaswith 32/38 inch are 92 inch and 57 inch respectively. if said frequencyis 400 kHz, the axial depth of investigation for the amplitude ratio andphase difference of the pair of antennas with 16/22 inch are 50 inch and47 inch respectively, and the axial depth of investigation for theamplitude ratio and phase difference of the pair of antennas with 32/38inch are 87 inch and 71 inch, respectively.

It can be seen from FIG. 3 to FIG. 8 that, as the formation resistivitycontrast increases, the variation in the amplitude ratio response orphase difference response compared with the variation in the position ofthe formation boundary is flater. As the formation resistivity contrastor the distance between one transmitting antenna and one receivingantenna increases, the axial depth of investigation of said loggingapparatus increases. In the formation having the same resistivitycontrast, the axial depth of investigation of the amplitude ratio curveof the same one antenna pair is greater than the axial depth ofinvestigation of the phase difference curve thereof.

During drilling forward by the drilling apparatus, the presence of aformation boundary or oil/water interface may be determined throughmeasuring in real-time the variations in the amplitude ratio or phasedifference by the apparatus for logging while drilling according to thepresent invention, whereby the drill tool can be controlled to passthrough the best location of the petroleum reservoir. If the amplituderatio and phase difference measured by the apparatus for logging whiledrilling do not vary during drilling forward by the drilling apparatus,it means that the readings of the amplitude ratio and phase differencegenerated by said apparatus for logging while drilling are substantiallyconstant, it indicates that there does not exist a formation withlow-resistivity in front of the apparatus for logging while drilling. Ifthe readings of the amplitude ratio and phase difference are no longer aconstant during drilling forward, it possibly indicates that a formationwith low-resistivity appears in front of the drilling apparatus, and itis necessary to timely adjust the trajectory of the borehole to avoiddrilling into the formation with low-resistivity. Consequently, thedrilling apparatus can be always disposed in the oil-containing targetformation with high-resistivity, thus can achieve the prediction offormation boundary before drilling and accurate geo-steering.

It will be appreciated by those skilled in the art that, although thepresent invention describes the preferred embodiment with respect topetroleum drilling, the apparatus and method for well logging accordingto the present invention are not limited to the technical field ofpetroleum drilling, and they can be further broadly adapted to coalmining, mining and other drilling industries.

Hereinafter, a data processing device for implementing the abovediscussed method for well logging according to a preferred embodiment ofthe invention will be set forth later in detail.

As shown in FIG. 14, the data processing device according to theinvention preferably comprises: means for selecting the first and secondmeasurement points 1400, means for calculating the first amount ofvariation in amplitude ratio and in phase difference 1401, means forcalculating the second amount of variation in amplitude ratio and inphase difference 1402, means for determining the first selectable pointof homogeneous formation 1403, means for determining the secondselectable point of homogeneous formation 1404, memory 1405, means forderiving essential value 1406, means for deriving standard value 1407,means for setting layer-out threshold 1408, means for selecting thethird through n^(th) measurement points and calculating the amount ofvariation in amplitude ratio and phase difference 1409, and means fordetermining the presence of a formation with low-resistivity 1410.

Wherein, said means for selecting the first and second measurementpoints 1400 selects two sequential measurement points (i.e. the firstmeasurement point and the second measurement point), and instructs theapparatus for well logging to take at least two sequential measurementsat each of the selected measurement points.

Said means for selecting the first and second measurement points 1400 iscoupled to the means for calculating the first amount of variation inamplitude ratio and in phase difference 1401 and the means forcalculating the second amount of variation in amplitude ratio and inphase difference 1402, respectively.

The means for calculating the first amount of variation in amplituderatio and in phase difference 1401 is configured for calculating theamount of variation in amplitude ratio ΔAtt and the amount of variationin phase difference ΔPSD of the induced electromotive force between thefirst receiving antenna and the second receiving antenna along the axialdirection of the apparatus for well logging from the at least twosequential measurements at the first measurement point.

The means for calculating the second amount of variation in amplituderatio and in phase difference 1402 is configured for calculating theamount of variation in amplitude ratio ΔAtt and the amount of variationin phase difference ΔPSD of the induced electromotive force between thefirst receiving antenna and the second receiving antenna along the axialdirection of the apparatus for well logging from the at least twosequential measurements at the second measurement point.

The means for determining the first selectable point of homogeneousformation 1403 is coupled to said means for calculating the first amountof variation in amplitude ratio and in phase difference 1401, and isconfigured for determining whether the first amount of variation inamplitude ratio ΔAtt and the amount of variation in phase differenceΔPSD at the first measurement point are within their respective presetthreshold range. If yes, the first measurement point is stored in thememory 1405 as the first selectable point of homogeneous formation. Ifnot, the means for selecting the first and second measurement points1400 is instructed to reselect another two measurement points.Preferably, the preset threshold range for the amount of variation inamplitude ratio may be set to 0-0.03 dB or other appropriate presetrange as desired, and the preset threshold range for the amount ofvariation in phase difference may be set to 0°-0.1° or other appropriatepreset range as desired.

The means for determining the second selectable point of homogeneousformation 1404 is coupled to said means for calculating the secondamount of variation in amplitude ratio and in phase difference 1402, andis configured for determining whether the second amount of variation inamplitude ratio ΔAtt and in phase difference ΔPSD at the secondmeasurement point are within their respective preset threshold range. Ifyes, the second measurement point is stored in memory 1405 as the secondselectable point of homogeneous formation. If not, the means forselecting the first and second measurement points 1400 is instructed toreselect another two measurement points.

The means for deriving essential values 1406 is coupled to the memory1405, and is configured for determining the amplitude ratio essentialvalue Δtt0 and phase difference essential value PSD0 of the signalresponse generated by the apparatus for well logging, which correspondsto the formation resistivity of the measured target formation withhigh-resistivity. According to a preferred embodiment, the means forderiving essential values 1406 takes the average value or mean squareroot of the multiple measurements of the amplitude ratio of the inducedelectromotive force between the first receiving antenna and the secondreceiving antenna measured at both of the first and second selectablepoints of homogeneous formation as the amplitude ratio essential valueAtt0. In a similar way, the means for deriving essential values 1406takes the average value or mean square root of the multiple measurementsof the phase difference measured at both of the first and secondselectable points of homogeneous formation as the phase differenceessential value PSD0.

Preferably, said means for deriving essential values 1406 may calculatethe formation resistivity, amplitude ratio and phase difference at saidfirst and second selectable points of homogenous formation by usingdyadic Green's function of magnetic dipole source with method ofrecursive matrix. As appreciated by those skilled in the art, the meansfor deriving essential values 1406 also may calculate the formationresistivity, amplitude ratio and phase difference at said first andsecond selectable points of homogenous formation by using other priorfunction or algorithm.

The means for deriving standard values 1407 is coupled to said means forderiving essential values 1406 and memory 1405, and is configured forderiving and storing the standard value corresponding to the formationresistivity of the measured target formation with high-resistivity.According to a preferred embodiment, the means for deriving standardvalues 1407 is configured for comparing said amplitude ratio essentialvalue Att0 and phase difference essential value PSD0 of the measuredhigh-resistivity target formation with the corresponding predeterminedeigenvalues of various types of formations. Then, the eigenvalues of thetype of formation closest to said amplitude ratio essential value Att0and phase difference essential value PSD0 can be selected as theamplitude ratio standard value and phase difference standard valuecorresponding to the formation resistivity of the measuredhigh-resistivity target formation. The amplitude ratio standard valueand phase difference standard value are stored in the memory 1405.

The means for setting layer-out threshold 1408 is coupled to the meansfor deriving standard values 1407 and memory 1405, and is configured forsetting a layer-out threshold of the measured target formation withhigh-resistivity. According to a preferred embodiment, the means forsetting layer-out threshold 1408 sets the layer-out threshold of saidmeasured high-resistivity target formation according to the amplituderatio standard value and phase difference standard value correspondingto the formation resistivity of said measured target formation.Thereafter, the layer-out threshold can be stored in said memory 1405.

Specifically, when the apparatus for well logging approaches a formationboundary with low-resistivity, the amplitude ratio and phase differenceof the induced electromotive force between the first receiving antennaand the second receiving antenna in the axial direction of saidapparatus for well logging will vary. The closer the apparatus for welllogging approaches the boundary with low-resistivity, the larger thedifference between said actually measured amplitude ratio as well asphase difference and the amplitude ratio standard value as well as thephase difference standard value are. When the amount of variation in theamplitude ratio and phase difference reaches or exceeds a preset value,it is normally deemed that a formation with low-resistivity appears infront of the apparatus for well logging. Said preset value is named asthe layer-out threshold herein.

It is noted that the layer-out threshold for different measuredformations may be set to different preset values by those skilled in theart according to the characteristics of the actually measured formationsand measurement conditions. Generally, the layer-out threshold may bederived from the resistivity contrast between the two formations of thecurrently measured formation and the axial forward formation.Preferably, no matter how the resistivity contrast between the twoformations of the currently measured formation and the axial forwardformation is like, the layer-out threshold may be set to be 1%-30% ofthe amplitude ratio standard value or phase difference standard value.Further preferably, when said resistivity contrast is 1/10, thelayer-out threshold may be preferably set to be 10% of said amplituderatio standard value or phase difference standard value. Theaforementioned manner of determining the layer-out threshold and thespecific value of the layer-out threshold are for illustrative purposeonly rather than for restrictive purpose, and those skilled in the artmay select appropriate values in other ways according to practice.

The means for selecting the third through n^(th) measurement points andcalculating the amount of variation in amplitude ratio and phasedifference 1409 is configured for proceeding to select a nextmeasurement point, taking at least two measurements at the nextmeasurement point, and calculating the amount of variation in amplituderatio ΔAtt and the amount of variation in phase difference ΔPSD of theinduced electromotive force between the first receiving antenna and thesecond receiving antenna along the axial direction of the apparatus forwell logging at such a measurement point.

The means for determining the presence of the formation withlow-resistivity 1410 is coupled to the memory 1405, the means forsetting layer-out threshold 1408, and the means for selecting the thirdthrough n^(th) measurement points and calculating the amount ofvariation in amplitude ratio and phase difference 1409.

According to the preferred embodiment, the means for determining thepresence of the formation with low-resistivity 1410 comprises a unit fordetermining the occurrence of layer-out 14101, which is configured fordeciding whether the amount of variation in amplitude ratio MU and theamount of variation in phase difference ΔPSD at the current measurementpoint calculated by said means for selecting the third through n^(th)measurement points and calculating the amount of variation in amplituderatio and phase difference 1409 are greater than the layer-outthreshold. If yes, it is determined that there is a low-resistivityformation in front of the apparatus for well logging; If not, the amountof variation in amplitude ratio ΔAtt and the amount of variation inphase difference ΔPSD at the current measurement point are stored in thememory 1405.

According to another preferred embodiment, the means for determining thepresence of the formation with low-resistivity 1410 further comprises aunit 14102 for determining the number of measurement points as well as aunit 14103 for determining the tendency of variation in amplitude ratioand phase difference.

The unit for determining the number of measurement points 14102 isconfigured for determining whether the currently selected measurementpoint is the preset n^(th) measurement point when the unit fordetermining the occurrence of layer-out 14101 determines that the amountof variation in amplitude ratio ΔAtt and the amount of variation inphase difference ΔPSD at the current measurement point are not greaterthan the layer-out threshold. if not, the means for selecting the thirdthrough n^(th) measurement points and calculating the amount ofvariation in amplitude ratio and phase difference 1409 is instructed toproceed to select a next measurement point and to calculate the amountof variation in amplitude ratio ΔAtt and the amount of variation inphase difference ΔPSD; or else if the currently selected measurementpoint is the preset n^(th) measurement point, the unit for determiningthe tendency of variation in amplitude ratio and phase difference 14103is instructed to determine the tendency of variation in amplitude ratioand the tendency of variation in phase difference according to thepreviously stored amount of variation in amplitude ratio ΔAtt and theamount of variation in phase difference ΔPSD at each measurement point(i.e. the third, fourth, fifth, . . . , n^(th) measurement point).

As discussed before, said number ‘n’ is preset by those skilled in theart based on the characteristics of the measured formation and themeasuring speed. For example, if the measured formation is a softerformation (e.g. sand rocks in a coastal area), n can be relativelysmaller, while if the measured formation is a harder formation (e.g.shale rocks), n can be relatively larger. Typically, for a generalformation, n may be preferably preset to be 20-30, but the inventionnever to be limited to such a range of value, and other appropriatevalues may be preset for n.

According to a yet further embodiment, the means for determining thepresence of the formation with low-resistivity 1410 further comprises afirst tendency determining unit 14104 which is configured fordetermining whether the tendency of variation determined by said unit14103 is that the amount of variation in amplitude ratio and the amountof variation in phase difference maintain a progressive increase fromthe third measurement point to the n^(th) measurement point (i.e. theamount of variation in amplitude ratio and the amount of variation inphase difference at the (m+1)^(th) measurement point are greater thanthe amount of variation in amplitude ratio and the amount of variationin phase difference at the M^(th) measurement point, wherein m=1, 2, . .. , n−1); If yes, it is determined that a formation with low-resistivityappears in front of said apparatus for well logging.

According to another further embodiment, the means for determining thepresence of the formation with low-resistivity 1410 further comprises asecond tendency determining unit 14105 which is configured fordetermining whether the tendency of variation maintain an approximatelyprogressive increase from the third measurement point to the n^(th)measurement point in the case of the determination of the first tendencydetermining unit 14104 being negative. If the tendency of variation doesmaintain an approximately progressive increase, it is also determinedthat a formation with low-resistivity is in front of said apparatus forwell logging; or else if the tendency does not maintain an approximatelyprogressive increase, it is determined that no formation withlow-resistivity appears in front of said apparatus for well logging.

As mentioned above, the ‘approximatively progressive increase’ hereinmeans that, although there are some ripples in the tendency of variation(in other words, the amount of variation in amplitude ratio and theamount of variation in phase difference at certain measurement point aresmaller than the amount of variation in amplitude ratio and phasedifference at the immediately previous measurement point), there are forexample at least 70% of the measurement points to maintain the tendencyof progressive increase. Said percentage also can be preset by thoseskilled in the art according to practice, and the percentage of 70% isonly for illustrative rather than for restrictive.

Please note that the preferred embodiments of the invention may beimplemented in any one of or the combination of hardware, software,firmware. In the various embodiment(s), the device components areimplemented by software or firmware stored in the memory and executed byan appropriate instruction execution system. If it is implemented inhardware, for example in some embodiments, the device components may beimplemented by any one of or the combination of the following techniqueswell-known by those skilled in the art: discrete logic circuit(s) havinga logic gate for performing logic function on data signals, anapplication-specific integrated circuit (ASIC) comprising an appropriatecombinational logic gate, programmable gate array(s) (PGA), afield-programmable gate array (FPGA) and so on.

Software components may include an ordered list of the executableinstructions for performing logic function, which may be embodied in anycomputer readable medium to be used by or in connection with aninstruction execution system, apparatus or device. Said instructionexecution system, apparatus or device is, for example, a computer-basedsystem, a system containing a processor, or other system that can obtaininstructions from the instruction execution system, apparatus or deviceand can execute said instructions. Besides, the scope of the presentdisclosure includes a function of embodying one or more embodiments inthe logic embodied in the medium composed of hardware or software.

The embodiments of the present disclosure have been disclosed for thepurpose of illustration. They do not intend to be exhaustive or restrictthe present disclosure to the disclosed precise forms. According to thedisclosure above, many variations and modifications of the embodimentsherein are apparent for those skilled in the art. It is noted that theabove examples do not intend to be restrictive. Additional embodimentsof apparatuses, methods and devices comprising many of the aforesaidfeatures may be further anticipated. The other apparatuses, methods,devices, features and advantages of the present disclosure are even moreapparent to those skilled in the art after making reference to thedetailed description and accompany figures. It is intended that all ofsuch other apparatuses, methods, devices, features and advantages areincluded in the protection scope of the invention.

Unless specified otherwise, conditional languages such as “be able to”,“can”, “possibly”, “may” and the like generally intend to indicate thatsome embodiments may but not necessarily comprise some features,elements and/or steps. Therefore, such conditional languages generallydo not intend to give a hint for requiring that one or more embodimentshave to comprise features, elements and/or steps.

The illustrative block diagrams and flow charts depict process steps orblocks that may represent modules, segments, or portions of code thatinclude one or more executable instructions for implementing specificlogical functions or steps in the process. Although the particularexamples illustrate specific process steps or acts, many alternativeimplementations are possible and commonly made by simple design choice.Acts and steps may be executed in different order from the specificdescription herein, based on considerations of function, purpose,conformance to standard, legacy structure, and the like.

1. A method for well logging, comprising: (a) a step of selectinghomogeneous measurement point, wherein an apparatus for well logging isconfigured to select two sequential measurement points to take at leasttwo sequential measurements at each measurement point; (b) determiningwhether said selected two sequential measurement points may serve asselectable points of homogeneous formation according to the measurementsat said two sequential measurement points, if yes, then proceeds to step(c); (c) deriving the amplitude ratio essential value and phasedifference essential value of the signal response generated by theapparatus for well logging, which corresponds to the formationresistivity of the measured target formation, from said two selectablepoints of homogeneous formation; (d) deriving the amplitude ratiostandard value and phase difference standard value corresponding to theformation resistivity of the measured target formation from saidamplitude ratio essential value and phase difference essential value;(e) setting a layer-out threshold of the formation for said measuredtarget formation according to said amplitude ratio standard value andphase difference standard value. (f) selecting a next measurement pointto take at least two measurements at said next measurement point; (g)deciding whether the amount of variation in amplitude ratio and/or theamount of variation in phase difference of the induced electromotiveforce between a pair of receiving antennas of the apparatus for welllogging at the current measurement point are greater than said layer-outthreshold; if yes, then proceed to step (h); (h) it is determined that aformation with low-resistivity appears in front of the apparatus forwell logging.
 2. A method for well logging according to claim 1, whereinsaid step (b) further comprises: returning to the step (a) to reselectanother two sequential measurement points if either of the selectedmeasurement points cannot serve as a selectable point of homogeneousformation.
 3. A method for well logging according to claim 1, whereinsaid step (b) further comprises: if the amount of variation in amplituderatio and the amount of variation in phase difference of the inducedelectromotive force between a pair of receiving antennas of saidapparatus for well logging at one of the selected measurement points arewithin their respective preset threshold range, said one of the selectedmeasurement point may serve as a selectable point of homogeneousformation.
 4. A method for well logging according to claim 1, whereinsaid step (c) further comprises: the average value or mean square rootof the multiple measurements of amplitude ratio of the inducedelectromotive force between said pair of receiving antennas measured atsaid two selectable points of homogeneous formation is considered assaid amplitude ratio essential value; and the average value or meansquare root of the multiple measurements of the phase difference of theinduced electromotive force between said pair of receiving antennasmeasured at said two selectable points of homogeneous formation isconsidered as said phase difference essential value.
 5. A method forwell logging according to claim 4, wherein, in said step (c), thecorresponding formation signal responses including induced electromotiveforce, amplitude ratio and/or phase difference associated with theformation resistivity at said two selectable points of homogenousformation are calculated by dyadic Green's function of magnetic dipolesource with method of recursive matrix.
 6. A method for well loggingaccording to claim 1, wherein, said step (d) further comprises:comparing said amplitude ratio essential value and phase differenceessential value with the corresponding preset eigenvalues of varioustypes of formations, and selecting the eigenvalues of the type offormation closest to said amplitude ratio essential value and phasedifference essential value as the amplitude ratio standard value andphase difference standard value corresponding to the formationresistivity of the measured target formation.
 7. A method for welllogging according to claim 1, wherein, said method for well loggingfurther comprises: Step (i), when the determination of step (g) isnegative, storing the amount of variation in amplitude ratio and amountof variation in phase difference at the current measurement point, anddetermining whether the currently selected measurement point if thepreset n^(th) measurement point, if not, returning to step (f), whereinsaid n is a positive integer greater than
 4. 8. A method for welllogging according to claim 7, wherein said method for well loggingfurther comprises: step (j), when it is determined in said step (i) thatthe currently selected measurement point is said preset n^(th)measurement point, the tendency of variation in amplitude ratio and thetendency of variation in phase difference are determined according tothe previously stored amount of variation in amplitude ratio and amountof variation in phase difference at each measurement point.
 9. A methodfor well logging according to claim 8, wherein said method for welllogging further comprises: step (k), if the tendency of variation isthat the amount of variation in amplitude ratio and amount of variationin phase difference maintain a progressive increase or approximativelyprogressive increase from the third measurement point to the n^(th)measurement point, it is determined that a formation withlow-resistivity appears in front of said apparatus for well logging; orelse if the tendency of variation does neither maintain a progressiveincrease nor maintain an approximatively progressive increase, it isdetermined that no formation with low-resistivity appears in front ofsaid apparatus for well logging.
 10. A data processing device, whereinsaid data processing device comprises: means for determining selectablepoints of homogeneous formation, which is configured to determinewhether both of the two sequential measurement points currently selectedby an apparatus for well logging may serve as selectable points ofhomogeneous formation or not; means for deriving essential values, whichis configured to derive the amplitude ratio essential value and phasedifference essential value of the signal response generated by theapparatus for well logging from said two selectable points ofhomogeneous formation when it is determined that the two sequentialmeasurement points currently selected may serve as selectable points ofhomogeneous formation; wherein the amplitude ratio essential value andphase difference essential value correspond to the formation resistivityof the measured target formation; means for deriving standard values,which is configured to derive the amplitude ratio standard value andphase difference standard value corresponding to the formationresistivity of the measured target formation from said amplitude ratioessential value and phase difference essential value, means for settinglayer-out threshold, which is configured to set a layer-out threshold ofthe formation for said measured target formation according to saidamplitude ratio standard value and phase difference standard value;means for selecting the third through the n^(th) measurement points andcalculating the amount of variation in amplitude ratio and phasedifference, which is configured to select a next measurement point totake at least two measurements at said next measurement point, andcalculate the amount of variation in amplitude ratio and the amount ofvariation in phase difference of the induced electromotive force betweena pair of receiving antennas of the apparatus for well logging at thecurrent selected measurement point; and means for determining thepresence of the formation with low-resistivity, which comprises a unitfor determining the occurrence of layer-out, said unit is configured todecide whether the amount of variation in amplitude ratio and/or theamount of variation in phase difference at the current selectedmeasurement point are greater than said layer-out threshold; if yes, itis determined that a formation with low-resistivity appears in front ofthe apparatus for well logging.
 11. A data processing device accordingto claim 10, wherein said means for determining selectable points ofhomogeneous formation (1403, 1404) is configured for determining whetherthe amount of variation in amplitude ratio and the amount of variationin phase difference of the induced electromotive force between a pair ofreceiving antennas of said apparatus for well logging at the selectedmeasurement points are within their respective preset threshold range,if yes, the measurement point under determining may serve as aselectable point of homogeneous formation.
 12. A data processing deviceaccording to claim 10, wherein, if said means for determining selectablepoints of homogeneous formation (1403, 1404) determines that any one ofthe selected measurement points may not serve as a selectable point ofhomogeneous formation, it indicates the apparatus for well logging toreselect another two sequential measurement points.
 13. A dataprocessing device according to claim 10, wherein said means for derivingessential values (1406) is further configured to: take the average valueor mean square root of the multiple measurements of amplitude ratio ofthe induced electromotive force between said pair of receiving antennasmeasured at said two selectable points of homogeneous formation as saidamplitude ratio essential value; and take the average value or meansquare root of the multiple measurements of the phase difference of theinduced electromotive force between said pair of receiving antennasmeasured at said two selectable points of homogeneous formation as saidphase difference essential value.
 14. A data processing device accordingto claim 10, wherein said means for deriving standard values (1407) isfurther configured to compare said amplitude ratio essential value andphase difference essential value with the corresponding preseteigenvalues of various types of formations, and to select theeigenvalues of the type of formation closest to said amplitude ratioessential value and phase difference essential value as the amplituderatio standard value and phase difference standard value correspondingto the formation resistivity of the measured target formation.
 15. Adata processing device according to claim 10, wherein said means fordetermining the presence of the formation with low-resistivity (1410)further comprises a unit for determining the number of measurementpoints (14102) and a unit for determining the tendency of variation inamplitude ratio and phase difference (14103); wherein said unit fordetermining the number of measurement points (14102) is configured fordetermining whether the currently selected measurement point is thepreset n^(th) measurement point when the unit for determining theoccurrence of layer-out (14101) determines that the amount of variationin amplitude ratio and amount of variation in phase difference at thecurrent measurement point are not greater than the layer-out threshold;if the currently selected measurement point is not the preset nthmeasurement point, the means for selecting the third through n^(th)measurement points and calculating the amount of variation in amplituderatio and phase difference (1409) is instructed to proceed to select anext measurement point and to calculate the amount of variation inamplitude ratio and the amount of variation in phase difference at thenext measurement point; or else if the currently selected measurementpoint is the preset n^(th) measurement point, the unit for determiningthe tendency of variation in amplitude ratio and phase difference(14103) is instructed to determine the tendency of variation inamplitude ratio and the tendency of variation in phase differenceaccording to the previously stored amount of variation in amplituderatio and amount of variation in phase difference at each measurementpoint.
 16. A data processing device according to claim 15, wherein themeans for determining the presence of the formation with low-resistivity(1410) further comprises a first tendency determining unit (14104) whichis configured for determining whether the tendency of variationdetermined by said unit for determining the tendency of variation inamplitude ratio and phase difference (14103) is that the amount ofvariation in amplitude ratio and amount of variation in phase differencemaintain a progressive increase from the third measurement point to thenth measurement point, if yes, it is determined that a formation withlow-resistivity appears in front of said apparatus for well logging. 17.A data processing device according to claim 16, wherein the means fordetermining the presence of the formation with low-resistivity (1410)further comprises a second tendency determining unit (14105) which isconfigured for determining whether the tendency of variation maintain anapproximately progressive increase from the third measurement point tothe n^(th) measurement point when the determination of the firsttendency determining unit (14104) is negative; if the tendency ofvariation does maintain an approximately progressive increase, it isdetermined that a formation with low-resistivity appears in front ofsaid apparatus for well logging; or else if the tendency does notmaintain an approximately progressive increase, it is determined that noformation with low-resistivity appears in front of said apparatus forwell logging.
 18. An apparatus for well logging, wherein said apparatusfor well logging comprises a drill collar body (12) and an array ofantennas, wherein said array of antennas comprises at least a pair oftransmitting antenna and receiving antenna, said transmitting antennaand receiving antenna are configured for generating a curve of axialforward depth of investigation.
 19. An apparatus for well loggingaccording to claim 18, wherein said array of antennas comprises fourtransmitting antennas T1 (11), T2 (14), T3 (13) and T4 (15), and fourreceiving antennas R1 (7), R2 (8), R3 (9) and R4 (10).
 20. An apparatusfor well logging according to claim 19, wherein the array of antennas isinstalled from the drill-collar-tail to the drill-head of the drillcollar body (12) in the following order: the receiving antenna R3, thetransmitting antenna T3, the transmitting antenna T1, the receivingantenna R1, the receiving antenna R2, the transmitting antenna T2, thetransmitting antenna T4, and the receiving antenna R4; the middle pointbetween said receiving antennas R1 and R2 is the measurement point, saidtransmitting antennas T1 and T2 are installed symmetrically about themeasurement point, and said transmitting antennas T3 and T4 areinstalled symmetrically about said measurement point; said receivingantennas R1 and R2 each have an installation angle of substantiallyzero; and said receiving antennas R3 and R4 are positioned on both endsof the drill collar body (12) and are symmetrical about said measurementpoint, and the installation angles of said receiving antennas R3 and R4are respectively set to be about 45° and −45°.